LEDs are becoming more and more valued because they provide benefits such as low power consumption, long lifetime, small volume and low cost. FIG. 1 is a circuit diagram of a conventional current regulator 10 for driving a white LED 20, which uses a first current source 12 to provide a reference current IREF, and a second current source to generate a direct current (DC) drive current ILED for the white LED 20 according to the reference current IREF. For the second current source, an operation amplifier 14 and two transistors 16 and 18 are so configured to be an low drop-out (LDO) current source, in which the transistor 16 has a drain connected to the first current source 12 to receive the reference current IREF, and a source connected to a ground terminal GND, the operational amplifier 14 has a non-inverting input connected to the drain of the transistor 16, and an output connected to a gate of the transistor 16, the transistor 18 has a drain connected to an inverting input of the operational amplifier 14 and the white LED 20, a source connected to the ground terminal GND, and a gate connected to the output of the operational amplifier 14, and therefore, the transistor 18 will mirror the reference current IREF in the transistor 16 to generate the drive current ILED for the white LED 20. FIG. 2 is a diagram showing a curve which describes the relationship between the reference current IREF and the dimming step of FIG. 1 in linear brightness control. This curve can be expressed byIREF=K1×STEP,   [Eq-1]where K1 is a constant and STEP is the number of the dimming step. FIG. 3 is a diagram showing a curve defined by the relationship between the DC drive current ILED and the dimming step based on the circuit of FIG. 1 and the equation Eq-1. Since the drive current ILED is generated by mirroring the reference current IREF, it will be also linearly proportional to the dimming step asILED=K2×IREF=K1×K2×STEP,   [Eq-2]where K2 is the size ratio of the transistors 16 and 18 of FIG. 1. FIG. 4 is a diagram showing a curve which describes the relationship between the luminous intensity (brightness) and the DC drive current of a white LED. As shown by this curve, when the DC drive current increases, the luminous intensity of a white LED increases in linear proportion thereto. In other words, it is a linear relationship between the brightness and the dimming step of a white LED.
Although in linear proportion to the dimming step, the brightness of a white LED varies nonlinearly to human eyes. FIG. 5 is a diagram showing a curve 30 which describes the relationship between the luminance and the lightness of a white LED, and a curve 32 which describes the relationship between the luminance and the lightness perceived by human eyes, in which the luminance Y in the vertical axis represents the actual luminance and the lightness L in the horizontal axis represents the luminance perceived by human eyes. For the curve 30, the lightness L in the horizontal axis may also represent the dimming step. Conventionally, the zero percentage value of the lightness L represents a fully black status, and the hundred percentage value of the lightness L represent a fully white status. Under ideal condition, each time the luminance Y of a white LED varies by ΔY, human eyes should perceive a lightness variation ΔL following the curve 30. However, because of the complex structure of human eyes, the real lightness variation ΔL perceived by human eyes follows the curve 32, in which a significant variation in the luminance Y is necessary for human eyes to perceive a variation in the lightness L where the luminance Y is high, and conversely, where the luminance Y is low, a slight variation in the luminance Y is sufficient to produce a perceptible variation in the lightness L. Therefore, each time the dimming step is changed by one to adjust the luminance Y of a white LED by ΔY, a different amount of variation in the lightness L is perceived by human eyes. In other words, the dimming step of the current regulator 10 shown in FIG. 1 is nonlinearly proportional to the lightness L perceived by human eyes. FIG. 6 is a diagram showing the error of the luminance Y between the curves 30 and 32 of FIG. 5. As can been seen clearly in FIG. 6, under a same lightness L, a great difference exists between the luminance Y emitted by a white LED and that required by human eyes, especially when the lightness L is around 50.
In the current regulator 10, the dimming step must be appropriately selected in order for human eyes to perceive linear variation in lightness. In the low current region, a slight variation in the luminance of the white LED 20 is sufficient to cause a perceptible variation in lightness. As a result, the number of the dimming step that can be selected in the low current region is relatively small. Conversely, in the high current region, only a large variation in luminance of the white LED 20 leads to a perceptible variation in lightness, so that the number of the dimming step that can be selected in the high current region is relatively large. However, whether it is in the low current region or in the high current region, there will be always some dimming steps left unused and wasted. Moreover, selecting the appropriate dimming steps necessitate complex computation and adds to difficulty in system design.
Therefore, it is desired an apparatus and method for perceptually linear LED brightness control without specially selected dimming steps.